Semiconductors as radiation detectors are discussed in G. F. Knoll, Radiation Detection and Measurement, John Wiley and Sons, Second Edition, 1989, pp. 349. Knoll defines an Ohmic contact as "a nonrectifying electrode through which charges of either sign can flow freely. If two Ohmic contacts are fitted on opposite faces of a slab of semiconductor and connected to a detection circuit, the equilibrium charge carrier concentrations in the semiconductor will be maintained. If an electron or hole is collected at one electrode, the same carrier species is injected at the opposite electrode to maintain the equilibrium concentrations in the semiconductor."
Regarding semiconductor diode detectors, Knoll states on page 350: "The steady-state leakage currents that are observed using Ohmic contacts are too high, even with the highest resistivity material available . . . . Instead, noninjecting or blocking electrodes are universally employed to reduce the magnitude of the current through the bulk of the semiconductor. If blocking electrodes are used, charge carriers initially removed by the application of an electric field are not replaced at the opposite electrode, and their overall concentration within the semiconductor will drop after application of an electric field."
Possible use of cadmium telluride for detection of pulsed x-rays in medical tomography is described in E. N. Arkad'eva et al, Sov. Phys. Tech. Phys. 26(9), September 1981, pp. 1122-1125. Arkad'eva et al state on pp. 1123-1124: "We used an M-P-M structure with osmium contacts as a detector based on these crystals. . . The current from the x-ray excitation (the photocurrent) at the detector may be written in the steady-state regime in the form j=e.mu..DELTA.nE, where e is the electronic charge, .mu. is the carrier mobility, E is the electric field, .DELTA.n is the concentration of photocarriers created by the light, .DELTA.n=.beta.k .tau. I, where .beta. is the quantum yield (.beta.=1), k is the absorption coefficient for x-rays in the transducer, .tau. is the carrier lifetime, and I is the radiation intensity."
FIG. 2 of Arkad'eva et al graphs x-ray absorption at various energies for cadmium telluride crystals of thickness 1 mm (curve 1) and 2 mm (curve 2).
FIG. 3 of Arkad'eva et al graphs dark current (curves 1 and 2) and photocurrent (curves 1' and 2') as a function of the voltage across the device in continuous wave operation.
A method of manufacture for a high-resistance n-type CdTe crystal is described in K. Zanio, et al, IEEE Trans. Nucl. Sci., NS21, 315, 1974.
A method for attaching contacts to the CdTe crystal is described in R. E. Braithwaite et al, Solid State Electron., 23, 1091, 1980.
A preferred method for producing p-type CdTe is described in E. Janik and R. Triboulet, "Ohmic contacts to p-type cadmium telluride and cadmium mercury telluride", J. Phys. D.: Appl. Phys., 16 (1983), 2333-2340.
A. Rose, in Concepts in photoconductivity and allied problems, Interscience Publishers, London, 1963, defines an Ohmic contact as follows (section 8.3): "An Ohmic contact is one that supplies a reservoir of carriers freely available to enter the semiconductor as needed."
The disclosures of all the above publications are hereby incorporated by reference.